Filter elements of this kind are available commercially. Such filter elements are widely used in conjunction with a variety of fluid systems for filtering process fluids, pressure fluids such as hydraulic oil, as well as liquid fuels and lubricants for preparing fluid media and the like.
In many cases, only a limited amount of usable space is available in fluid systems, in which the filter elements are used, for installing or removing system parts that contain the relevant filter cartridge-type filters. On the other hand, a filtering surface of sufficient size provided by the filter element is required to filter correspondingly large fluid flows.
To provide a sufficiently large filtering surface, the known filter elements available on the market have a typically zig-zag-shaped folded or pleated filter medium composed of multiple layers of various filter materials. During manufacture, the filter medium is fed through a cutting device, in which the edge of the filter medium is cut to size before it is conveyed further to a folding machine, in which the zig-zag shape or the pleating, having a plurality of individual filter folds, is formed. During the further course of manufacture, the customized filter medium is separated into sections, which are shaped to form a tubular body, thereby forming the filter element.
In the standard filter element solution described above, all the filter folds routinely have the same insertion height. Depending on the flow-through conditions, the multiple filter folds disposed adjacent to one another may be displaced toward one another due to their flexibility or resilience, and thus come into direct contact with one another along their effective filtering surface. This displacement results in a type of “blocking” of the element material in this contiguous region, since the medium to be filtered is then no longer able to reach uninterruptedly all the filter folds of the element structure. The result is that the remaining filter folds spread apart from one another, are not blocked, and are increasingly perfused by the fluid to be cleaned of the particle contaminants. As a consequence, the flow velocity rises, and the surface load on these folds of the filter medium is increased. Since the multilayer filter medium is routinely formed from individual nonwoven filter medium of individual fibers, this load increase results in an increased discharge of fiber material from the fleece-composite material (migration), which, in turn, reduces the service life of the filter element.
One great challenge the filtration technology must now face is the fact that, due to environmental regulations, the hydraulic fluids to be cleaned using primarily such filter elements may no longer include any metal additives, in particular, any environmentally harmful zinc additives. As a result, the electrical conductivity of the hydraulic fluid is reduced. Due to this reduced conductivity, electrostatic charges, as they routinely occur during flow-through of the medium, can no longer be effectively dissipated via the hydraulic fluid, as in the past. As a result of this inability, discharge processes may occur in the filter element and may occur in the form of discharge flashes. Those flashes routinely destroy the sensitive filter medium in the same way that they promote the oil aging of the hydraulic fluid(s). Particularly in the case of in-tank applications of the filter element, in which the filter element is used in closed tank units, such discharge flashes may increase the danger of explosion.
To counter these effects, the prior art (DE 10 2004 005 202 A1) has proposed providing, in conjunction with a filter element having a filter medium, which extends between two end caps. Each end cap is connected to an assignable end region of the filter medium and is supported on at least one side by a support tube. At least one of the end caps and/or at least one end region of the filter medium includes a contacting device and/or each end cap itself or parts thereof are designed to be conductive, to thereby ensure a dissipation of the electrostatic charges routinely occurring during operation of the filter element. The charge generated on the filter medium as a result of the above described tribological effects may thus drain off at a ground point or ground site via the contact device and/or via each end cap. The “controlled dissipation” in this regard has proven to be very effective. However, maintaining the upstream contact device requires increased material and manufacturing expenditure, which, accordingly, reflects negatively in terms of the manufacturing costs of the element.
An alternative approach has been described in WO 2009/089891 A1, which shows a filter element solution without the use of a contact device. In this known solution, a manufacturing material is selected for the filter medium, the potential difference of which to the fluid to be cleaned is minimal, depending on the respective selected cleaning task. In particular, filter solutions in this case are addressed, in which parts of the filter medium exhibit such different potentials relative to one another and/or to the fluid to be cleaned (hydraulic fluid), that, at least in part, they cancel each other out during the filtration operation, or in which a targeted discharge back into the hydraulic fluid is sought, or in which a return of electrical charge to the entire filter medium is provided using a charge equalization layer in the filter media system. Thus, with this known solution, estimating the potential produced between two interacting components, i.e., between oil and filter medium, in a normal application, is possible in accordance with a known electrical voltage series for various filter materials provided for the filter medium (filter fleece). In this known, very advantageous solution, very little charge is generated, in principle, so that the problem of having to discharge the former at a ground point as shown in the above description, via a discrete contact device, does not even arise. Nevertheless, here too, when selecting the filter insert materials in question, based on the aforementioned tribological voltage series, a correspondingly high expense must be incurred, both from the standpoint of pre-development as well as from the standpoint of material supply, to obtain positive results, which again increases the manufacturing costs of the filter element.
In addition, all of the known solutions described above have in common the fact that the disadvantageous “blocking” effect may occur as a result of displacement of free filter fold ends due to the flow-through of the fluid.